Advances in integrated circuit (IC) technology has resulted in shrinking the channel lengths of metal-oxide-semiconductor (MOS) transistors which increases the switching speed of these transistors. Heightened governmental regulation is reducing the electro-magnetic interference (EMI) allowance which may be generated by an electronic system. Furthermore, systems require increased electro-static discharge (ESD) tolerance. As the channel length is reduced, the transistors get faster and conduct more current. Increasing transistor width or holding the widths to pre-shrink sizes means that the peak source and sink capability of the transistors in an IC output buffer is increased. This directly increases the amount of electromagnetic radiation which contribute to EMI and also increase the chances of latchup in the output buffer. Furthermore, for the same drive requirements the output driver transistor can be smaller which may result in ESD protection being compromised.
In a prior art buffer, when the buffer is enabled and an output terminal transitions from a LOW to a HIGH or a HIGH to LOW, output capacitance (parasitic or otherwise) which is connected to the output terminal is suddenly charged or discharged. This may cause a large amount of current to flow through the power and ground lines. Due to internal device routing line resistance and printed circuit board (PCB) trace resistance on the power and ground lines, the following conditions may happen:
i) the voltage on the power line tends to dip momentarily during a low to high transition of the output; and
ii) the voltage on the ground line rises momentarily during a high to a low transition on the output.
Both the conditions (i) and (ii) above are undesirable for the reasons listed below:
a) If the voltage dips below a diode drop from power line (VCC), there could be large substrate currents therefore increasing the chances of a latchup;
b) If the voltage rises above a diode drop from ground line (VDD) there could be substrate currents again increasing the chances of a latchup; and
c) Even if sufficient precautions were taken to prevent above conditions, the dip/rise on the power/ground lines radiates noise.
A strong output transistor has a low characteristic impedance. Typically, an impedance mismatch occurs between the output transistor and the printed circuit board traces which results in ringing. FIG. 1 illustrates an output signal 102 of a typical prior art output buffer which has overshoot/ringing 104 and undershoot/ringing 106. Ringing contributes to EMI and is therefore undesirable. Prior art FIG. 1 also illustrates output current 107 of a typical prior art output buffer which has current spikes 108 and 109. Current spikes 108 and 109 may cause undesirable fluctuations on power and ground.
With the EMC radiation regulation requirements getting more and more difficult to meet, it becomes a requirement to create more innovative circuits. Accordingly, there is needed an output buffer which overcomes these EMI and ESD problems.
Other objects and advantages will be apparent to those of ordinary skill in the art having reference to the following figures and specification.